In general, a magnetic recording medium comprising a non-magnetic support having thereon a magnetic layer containing ferromagnetic powders, such as .gamma.-iron oxide, Co-containing iron oxide, CrO.sub.2 and ferromagnetic metal powders each dispersed in a binder, is used for tape magnetic recording mediums for, for example, audio, video, and computer and disc magnetic recording mediums for, for example, floppy discs. The non-magnetic support used for this magnetic recording medium is usually made of polyethylene terephthalate, polyethylene naphthalate and the like. These supports have a high mechanical strength and an excellent anti-solvent property since they are extended and highly crystallized.
The magnetic layer, provided by applying a coating solution prepared by dispersing ferromagnetic powders in a binder on a non-magnetic support, is fragile due to a high filling degree of the ferromagnetic powders and a small breaking extension; and therefore, the magnetic layer formed without providing an undercoating layer is readily broken with mechanical strength applied thereto, and peeled off from the non-magnetic support in several instances. In order to strongly adhere the magnetic layer to the non-magnetic support, it is the easiest and safest approach to provide an undercoating layer, e.g., as used in applying paint. With respect to this undercoating, various attempts have been made in the past; namely, JP-B-47-22071 (the term "JP-B" as used herein means an examined Japanese patent publication), JP-B49-10243, JP-A-52-42703 ( the term "JP-A" as used herein means an unexamined published Japanese patent application), and JP-A-59-19230. However, these were not satisfactory.
Further, the approaches in which polyester resins were used as the resin for an undercoating layer in order to improve the adherence property with polyethylene terephthalate were disclosed in JP-B-62-37451, JP-A-56-87233, JP-A-60-11358, JP-A-60-19522, JP-A-60-21250, JP-A-61-264510, and JP-A-1-245421.
In view of recent developments in the art, a greater smoothness is required for the magnetic recording media for which a high recording density is requested; e.g., S-VHS, 8 mm video, Hi-8 and Hi-vision. Further, in keeping step with the trend toward compactness, a thinner magnetic recording medium and a thinner magnetic layer are required.
In order to achieve a high recording density, a high dispersion of ferromagnetic fine powders is needed. Consequently, the binders into which a hydrophilic group, such as COOH, SO.sub.3 Na and a group having a phosphoric acid group, are used for the magnetic layer; and to dissolve the binders, the solvents themselves (which are used for magnetic paint) have been partly switched from conventional solvents such as MIBK, toluene and MEK, to the solvents having a high hydrophilic property and dissolubility, such as cyclohexane.
When a conventional undercoating layer is applied to the magnetic recording medium, the undercoating layer is dissolved by the solvent contained in the magnetic paint, and is diffused into the magnetic layer during the period between coating and drying. Therefore, the adhesive strength between the magnetic layer and non-magnetic support is lowered. Further, in the case of a thin magnetic layer, an undercoating layer, which is unevenly swollen, lowers the smoothness of the magnetic layer. The problem has been clearly shown that the thinner and smoother the magnetic layer is, the more notably this phenomenon takes place and the electromagnetic characteristics are lowered.
The grain sizes of ferromagnetic powders (which become finer) together with the tendency toward high recording density, results in further hardening the magnetic layer and weakening the mechanical strength thereof. Further, the adhesive strength between the magnetic layer and non-magnetic support is lowered, and an increased dropout is due to a deteriorated durability and repeated running.
It has been found that a conventional undercoating layer cannot apply to such magnetic recording medium as used for high density recording.